Browsing by Author "Berger, Tilmann Georg"

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Phase transformations in interstitial Pd-B alloys
(2005) Berger, Tilmann Georg; Mittemeijer, Eric Jan (Prof. Dr. Ir.)
One, on the first view exotic interstitial phase, but as model system very interesting is the terminal palladium-rich solid solution PdBy. It is able to dissolve boron up to a B content corresponding to PdB0.25, the highest solid solubility of B in a transition metal. This solid solution is a high-temperature phase which is only stable above about 450°C but which can be retained at room temperature by quenching. The low-temperature regime consists of a miscibility gap, other two-phase areas and one or two low-temperature phases. The Pd host lattice experiences an extraordinarily large expansion upon dissolving boron. The observed lattice expansion upon incorporation of boron atoms on the octahedral interstices suggested large static displacements of the Pd atoms from their ideal coordinates in the crystal resulting in a large attenuation of diffracted Bragg intensities (Debye-Waller factor) with a characteristic dependence from the boron content. Neutron powder diffraction experiments were performed on a series of PdBy alloys to study these static displacements as a function of composition. Indeed significant displacements were observed for the PdBy alloys compared to those of pure Pd, for which no static displacements should occur. However, no further systematic interdependency between the interstitial content y and the magnitude of the static displacements was obtained. Comparison with isotypic defect-NaCl (interstitial) type transition metal nitrides and carbides indicated that the measured values for the static displacements of the PdBy alloys are exceptionally small. Additionally, the analysis of a compilation of data on static displacement parameters for these non-stoichiometric transition metal nitrides and carbides didn’t show the expected magnitude of the composition-dependent static displacement parameter either, in contrast to the currently accepted theories. The miscibility gap in the phase PdBy was reported in the range of 0.02 < y < 0.11 at 312°C with a critical point at 410°C (y = 0.065). An annealing time-resolved X-ray powder diffraction study of composition distributions is presented to trace the process of the decomposition of the solid solution upon annealing for various times in the field of the miscibility gap. For the analysis of the diffraction data a method was developed to fit the composition distribution of PdBy in partially decomposed samples by a discretised probability-density function for composition to the diffraction data. Three main component phases could be observed, first the initial solid solution with a phase content decreasing with increasing annealing time and secondly two boundary phases with increasing phase contents. It was shown that the decomposition into two solid solution phases is not complete even after the longest applied annealing times. One striking observation is that the observed final state after long annealing times with respect to the compositions of the boundary phases depends slightly on the boron content of the initial solid solution. One or two low-temperature phases have been reported in the composition range of 0.16 < y < 0.20 without providing convincing crystal structure information. The low-temperature phases occur in relatively small composition ranges because their formation is driven by ordering of boron atoms. The loss of symmetry as exhibited by the occurrence of superstructure reflections and splitting of fundamental reflections (those reflections already occurring for the solid solution) is difficult to observe on the basis of X-ray powder diffraction data alone, due to the low scattering power of the boron atoms for X-ray radiation leading to very weak superstructure reflections and due to the partially severe overlap between inequivalent fundamental reflections. Therefore, transmission electron microscopy and neutron powder diffraction were additionally employed to characterise the superstructure reflections. The monoclinic superstructures of the low temperature phases Pd6B and ‘Pd5B’ could be identified. The basic rule underlying the boron ordering in the fcc host lattice was determined to be avoidance of simultaneous occupation of nearest and in particular of next nearest octahedral interstices by B. The crystal structure information obtained by electron and neutron diffraction allowed a successful interpretation of the X-ray powder diffraction data. Many disagreements about the crystal structure and composition of the low-temperature phases and about the miscibility gap in the Pd-B system remained in spite of several works on that topic. In the light of the recent crystallographic investigations, a summary is given in this work leading to a revised Pd-B phase diagram, which explains nearly all observations from this and previous works.